Azobenzene Based Multiple Stimuli-responsive Supramolecular Hydrogels

Hydrogel is three-dimensional network structure containing large amounts of water,showing the property of flexible material,solid-like rheological behavior and expansion-shrinkage behavior.Hydrogel can be constructed not only by chemically crosslinked polymers but also by low molecular weight gelators or physically crosslinked polymers.Among them,the hydrogel which formed by low molecular weight compounds self-assemble via non-covalent interactions(including hydrogen bonds,hydrophobic interactions,?-?interactions,van der Waals forces,etc.),is called as supramolecular hydrogel.Recent years,it has attracted great interests in the fields of basic research and material application because of its rich structure and various construction methods.Stimuli-responsive materials refer to those materials that properties or functions can change with the change of external environment.Recent years,this kind of materials has received more and more attention from researchers.These external stimuli include temperature,light,mechanical force,pH and so on.The responses to these stimuli include color change,gel-sol transition,gel-gel transition,morphology changes and so on.Compared with traditional polymer gels,the dynamic and reversible nature of the non-covalent interactions endows supramolecular hydrogels easy to modulate and stimuli-responsive properties,and thus make them have extensive application prospects in the petroleum industry,cosmetics,organic electrons and drug release fields.Azobenzene compounds refer to the compounds formed by using azobenzene group to connect two benzene rings via covalent bond.They have cis-and trans-configurations,and are a typical photo-responsive material,can achieve reversible cis-trans isomerization under photo irradiation.During the construction of supramolecular hydrogels,if introduce azobenzene photoactive group to the gelator,we can achieve the construction of photo-responsive supramolecular hydrogels by taking advantage of the photo-induced cis-to-trans isomerization of azobenzene.Because photo irradiation will cause the change of molecular structure,charge and polarity,weaken the intermolecular interactions,lead to the dissociation or morphology change of assembly,and finally result in the transition of gel-sol.In this paper,we attempt to utilize Bola-form cationic surfactants containing two azobenzene groups to construct a kind of supramolecular hydrogels,and the supramolecular hydrogels based on low molecular weight gelators can respond to external stimuli.By modulating the hydrogels in many ways,we realize the construction of multiple stimuli-responsive supramolecular hydrogels.We expect that the supramolecular hydrogels with stimuli-responsive property will promote the development of smart materials.Firstly,a series of Bola-form surfactants containing two azobenzene groups were designed and synthesized by utilizing p-toluidine as the original material.They are separated by flexible chains,but the cationic heads are different.The molecular design is based on the following two considerations:firstly,we expect to utilize the reversible photo-induced cis-to-trans isomerization property of the azobenzene group to realize the construction of photo-responsive gels;Secondly,the hydrophobicity of the azobenzene is beneficial to its self-assembly in aqueous solution,which can support the formation of the supramolecular gel;finally,the solubility in aqueous solution can be increased by selecting polyethylene glycol chain as the connecting point.In conclusion,the molecular design not only ensures water solubility,but also facilitates self-assembly and the formation of the hydrogels.The chemical structures of the target compounds were characterized by ~1H NMR,ESI-MS,organic elemental analysis and FT-IR spectra.The results proved the chemical structures of the surfactants,with satisfactory purity.Secondly,the Bola-form surfactants obtained from the previous work show rich self-assembly behavior in aqueous solution,and the differences of the cationic heads lead to different gel behaviors and gel strengths.Among them,MIM-Azo and Py-Azo can form hydrogel,while TMA-Azo cannot form hydrogel even at a higher concentration,however,it can reduce the critical gelation concentration of MIM-Azo and Py-Azo.Firstly,the supramolecular hydrogels were prepared by heating-cooling method,and the formation of hydrogels was judged by tube inversion method.Secondly,the morphology of the hydrogels was characterized by scanning electron microscopy(SEM),and they present crosslinked fiber structure.Moreover,with the increase of concentration,the fibers begin to entangle.The microstructure of the hydrogels was observed by high resolution transmission electron microscopy,and they showed ordered layer structures,the interlayer spacing was consistent with the estimated molecular length.In addition,according to the results of powdered X-ray diffraction,we proposed the possible arrangement mode of molecule,and drawed the possible illustration of molecular arrangement.The mechanical strength of hydrogels was characterized by rheological tests.Finally,we investigated the multiple stimuli-responsive behaviors of the hydrogels.The obtained hydrogels can respond to light,temperature,chemicals and competitive guest molecules.The gel-to-sol transition can be caused not only by heating,but also by UV light irradiation.While a reverse process can be achieved by visible light irradiation.With the addition of cyclodextrins,the hydrogel transformed into sol,and the sol returned to the gel state with the addition of competitive guest molecules.Besides,the surfactant hydrogels exhibited stimuli-responsive behaviors to the addition of anions and cations based on different mechanisms,and the reverse adding order of guest molecules and cyclodextrins also led to reversible gel-sol transition.By combining light and chemical stimuli,the reversible gel-sol transition can be realized.This stimuli-responsive intelligent hydrogel shows potential application value in the field of drug release,environmental repair and so on.In conclusion,starting from the idea of constructing supramolecular hydrogels with Bola-form surfactants,a series of Bola-form surfactants containing two azobenzene groups with different cationic ends were designed and synthesized.The self-assembly structure of the supramolecular gel was characterized.Importantly,by adjusting the gel with light irradiation and the addition of chemical additives,we can realize the construction of multiple stimuli-responses of the supramolecular hydrogel.The work in this paper provides a new idea for the construction of stimuli-responsive supramolecular gels and is expected to promote the development of intelligent materials.